Introduction: A positive correlation between serum thyrotropin concentrations (TSH) and body mass index (BMI) has been demonstrated. Some studies not shown this, in euthyroid subjects, or observed only if thyroid autoimmunity (TA) is present or in non-smokers (NS). Leptin (Lp) could be the major link between BMI and TSH.

Design: To analyze the relationship between TSH, free thyroxine (FT4), Lp, TA (peroxidase and/or thyroglobulin antibodies) and smoking status in a representative sample of euthyroid, iodinesufficient, non-hospitalized population of Catalonia, with different BMI. Data collected included if each person smoked or had smoked. Glycemia and insulinemia were also determined and HOMA index, calculated.

Results: Eight hundred ninghty four adults (390 men) of 44.87±15.03 years and BMI 26.19±4.61 kg/m2 (17.0152.70) with normal TSH (0.333.96 mIU/l) and FT4 (0.871.90 ng/dl), and median urine iodine concentration 150.0 μg/l were studied. Lp correlated directly with BMI (P=0.00). There was no correlation between TSH and BMI. In men, TSH correlated directly with Lp (P=0.00) and in women, directly with Lp (P=0.00) and HOMA (P=0.03) and inversely, with FT4 (P=0.02). Only in smoker men, TSH correlated directly with Lp (P=0.01) and HOMA (P=0.02). In smoker women, TSH correlated directly with Lp (P=0.00) and in NS women, inversely with FT4 (P=0.04). The multivariate analysis showed that in men, the significant predictors of TSH variations are Lp, directly (P=0.02) and age, inversely (P=0.01). In women, the predictors are Lp and the presence of TA, directly (P=0.04; 0.00) and FT4, inversely (P=0.01).

Conclusions: Leptin is a predictor factor of TSH concentration variations, in euthyroid subjects. Other predictor factors are different in men and women. The smoking status can influence the relationship between TSH and some predictor factors. These data should be taken into account before drawing conclusions about the parameters that influence TSH concentrations in euthyroid people with different BMI.

Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.

Funding: This research did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector.